/*-
 * SPDX-License-Identifier: 0BSD
 *
 * Copyright (c) Robert Clausecker <fuz@FreeBSD.org>
 * Based on a publication by Daniel Lemire.
 * Public domain where applicable.
 *
 * Daniel Lemire, "Fast Random Integer Generation in an Interval",
 * Association for Computing Machinery, ACM Trans. Model. Comput. Simul.,
 * no. 1, vol. 29, pp. 1--12, New York, NY, USA, January 2019.
 */

#include <stdint.h>
#include <stdlib.h>

uint32_t
arc4random_uniform(uint32_t upper_bound)
{
        uint64_t product;

        /*
         * The paper uses these variable names:
         *
         * L -- log2(UINT32_MAX+1)
         * s -- upper_bound
         * x -- arc4random() return value
         * m -- product
         * l -- (uint32_t)product
         * t -- threshold
         */

        if (upper_bound <= 1)
                return (0);

        product = upper_bound * (uint64_t)arc4random();

        if ((uint32_t)product < upper_bound) {
                uint32_t threshold;

                /* threshold = (2**32 - upper_bound) % upper_bound */
                threshold = -upper_bound % upper_bound;
                while ((uint32_t)product < threshold)
                        product = upper_bound * (uint64_t)arc4random();
        }

        return (product >> 32);
}